1. Field of the Invention
The present invention relates to a fuel cell system, and more particularly, to a fuel cell system that can equally supply fuel to a plurality of electricity generating units which generate electricity by reacting fuel with air supplied from the outside.
2. Description of the Prior Art
A fuel cell is an electricity generating system that directly converts chemical reaction energy of hydrogen contained hydrocarbon substances such as methanol, ethanol and natural gas and an oxidizing agent into electric energy. This fuel cell typically comprises a polymer electrolyte membrane fuel cell (hereinafter referred to as a “PEMFC”) system and a direct methanol fuel cell (hereinafter referred to as a “DMFC”) system.
Generally, the PEMFC system comprises a stack that generates electric energy by a reaction of hydrogen and oxygen, and a reformer that generates hydrogen by reforming fuel. This PEMFC system has an advantageous effect of high energy density and high power. However, the PEMFC system requires careful handling of hydrogen gas and subsidiary facilities such as a fuel reformer for reforming methane or methanol and natural gas so as to produce fuel gas, i.e., hydrogen.
On the other hand, the DMFC system generates electricity by electrochemical reaction by directly supplying methanol fuel and an oxidizing agent, i.e., oxygen to the stack. This DMFC system has an advantageous effect in that energy density and power density are very high, and subsidiary facilities such as a fuel reformer and so on are not required because liquid fuel such as methanol is directly used, and fuel is easily stored and supplied.
In the DMFC system, the stack that actually generates electricity has a structure in which at least one unit cell including a membrane-electrode assembly (hereinafter referred to as a “MEA”) and a separator (or a bipolar plate) are stacked. The MEA is formed by interposing an electrolyte membrane between an anode electrode and a cathode electrode. Furthermore, each of the anode electrode and the cathode electrode is provided with a fuel diffusion layer for supplying and diffusing fuel, a catalyst layer for an oxidation/reduction reaction of fuel, and an electrode support.
The DMFC system can be variously formed depending on the arrangement structure of an electricity generating unit and the supply method of air. One type among them is a mono-polar type in which electricity generating unit is formed by arranging a plurality of unit cells on a plane. The mono-polar type is also referred to as a passive type or a semi-passive type because a pump is not used for supplying air.
The fuel cell system of the mono-polar type is formed in a plate form, and is formed such that a plurality of unit cells generating electricity by a reaction of fuel and air supplied from the outside are arranged on a plane. The plurality of unit cells constituting the electricity generating unit are provided with a cathode part to which air is supplied and an anode part to which fuel is supplied. The cathode electrode is exposed to air and is supplied with air by natural diffusion or convection effect of air. Furthermore, the anode part is supplied with fuel by a pump.
The unit cells are arranged on a plane, and thus are sequentially supplied with fuel from the unit cell formed on one side. Hence, a difference in the pressure of the supplied fuel is occurred depending on the position of the unit cell, and thus a difference in the amount of the supplied fuel is caused. Furthermore, the fuel cell system has a problem in that the power and efficiency thereof is lowered due to a difference in power produced in each unit cell.